1. Field of the Invention
The present invention relates to a method of evaluating a capillary pressure curve of the rocks of an underground reservoir from measurements on rock debris taken therefrom.
2. Description of the Prior Art
Laboratory measurements on cores or cuttings
Measurement of petrophysical parameters such as the permeability, the porosity and the capillary properties on rock fragments obtained while drilling a well through an underground formation is an interesting opportunity for operator companies to rapidly obtain a first petrophysical characterization of producing zones traversed by the well.
French Patent 2,809,821, filed by the assignee, describes a system of evaluating physical parameters such as the absolute permeability of porous rocks of an underground reservoir zone, from cuttings returned to the surface in the drilling mud. In an enclosure where the cuttings are dipped in a viscous fluid, some of this fluid is injected at a pressure that increases with time until a predetermined pressure threshold is reached, so as to compress the gas trapped in the pores of the rock. This injection stage is followed by a relaxation stage where injection is stopped. The pressure evolution during the injection process is modelled from initial values selected for the physical parameters of the cuttings. A computer adjusts the values iteratively so as to obtain the best possible agreement between the modelled pressure curve and the pressure curve really measured.
French patent application 02/0023, filed by the assignee, describes another method of evaluating physical parameters such as the absolute permeability and the porosity of the rocks of an underground reservoir zone, also from cuttings. An enclosure containing the rock fragments and filled with a viscous fluid communicates with a vessel containing the same fluid at a predetermined pressure so as to compress the gas trapped in the pores of the rock. The time of application of this pressure, according to whether it is short or long, allows measuring either of the pressure variation in the enclosure or the variation of the volume actually absorbed by the rock fragments. The pressure or volume evolution in the enclosure is then modelled from initial values selected for the physical parameters of the fragments, and the values of the physical parameters of the rock fragments are iteratively adjusted so that the modelled evolution best adjusts to the measured evolution of the physical parameter in the enclosure.
In the field of petrophysical study, the capillary pressure is also a very important datum for operators because it conditions:
the initial distribution of the fluids in the reservoir from the aquifer zone (referred to as WOC, for water-oil contact, by those skilled in the art) to the upper part of the reservoir (transition zone). According to the capillary pressure curve associated with a reservoir rock and the nature of the fluids in place, this transition zone can extend over many meters, which has an important effect on the determination of the accumulations in place,
the input pressure of a rock, which is particularly important for cap rocks. For example, for a gas storage tank, the input pressure of the cap rocks conditions directly the allowable overpressure in the storage levels without having leaks.
With the current techniques, the capillary pressure curve is obtained by means of laboratory measurements on reservoir cores. These methods are expensive because of the coring operations as well as the measurements on the cores, and the results are often available only several months after drilling.
Approaches to rapidly obtain the capillary pressure curve
However, there are alternative methods described in the literature for evaluating the capillary pressure curve rapidly, either during drilling or slightly later.
The most commonly used approach uses the mercury porosimetry technique for measuring the air/mercury capillary pressure curve Pc directly from cuttings. However, the obtained curve is significantly different from the reference curve obtained from cores with high wetting fluid saturations. Further, this approach is based on the use of mercury, which is extremely polluting and progressively forbidden by the law in many countries, which poses a major problem for applying this technique in the near future.
Another known method uses the Nuclear Magnetic Resonance (NMR) technique to rapidly estimate the capillary pressure curve from stratigraphic data measured in the well shortly after drilling. It is notably described in the following publications:
Bowers, M., A. et al.: “Prediction Of Permeability From Capillary Pressure Curves Derived With NMR”, 17 Sep. 1998,
Marshall, D., et al.: “Method For Correlating NMR Relaxometry And Mercury Injection Data”, SCA No. Society of Core Analysts International Symposium 1995,
Volokitin, Y., W. J. et al.: “A Practical Approach To Obtain 1st Drainage Capillary Pressure Curves From NMR Core and Log Data”, SCA No. Society of Core Analysts International Symposium 1999.
The NMR relaxation signal is first converted in terms of pore size distribution, then in terms of threshold size distribution, which allows calculation of a pseudo-capillary pressure curve. This approach has been tested on several samples of known curve Pc. The results show that a good agreement with the reference curves can only be obtained by means of a rigorous calibration stage to be carried out case by case according to the nature of the rocks studied. This calibration stage is necessary owing to the uncertainty on:
the NMR signal-pore size distribution conversion which depends on the value of the surface relaxivity which is variable according to the rocks, and
the pore size distribution-threshold size distribution conversion which depends on the nature of the rock and on the diagenesis process.
This approach is therefore not recommended in a predictive exploration context. In any case, it would not be applicable to cuttings.
Image analysis has also been the subject of work intended to obtain a curve Pc. The porous medium is first prepared in a form of a thin section photographed by a scanning electron microscopy or SEM. The image obtained is then analyzed so as to determine parameters representative of the proportion and of the shape of the voids in relation to the rock. In particular, it is possible to determine a threshold size distribution to reconstruct a pseudo-capillary pressure curve Pc. The main limitation of this method is the two-dimensional (2D) nature of the thin section, whereas the capillary pressure is by definition a three-dimensional (3D) property. Besides, this technique requires quite heavy conditioning, which is not really compatible with a result obtained slightly later. Image analysis could be applicable to cuttings but it would require careful calibration to acquire a good predictability.
Finally, it can be noted that the centrifuging technique is sometimes applied in the field to cuttings in order to extract the largest possible amount of drilling fluid from the rock to minimize pollutant discharges to the environment and to limit the cost by recycling the drilling fluid recovered. The inventors do not know of centrifugation of cuttings having been considered in order to determine capillary properties.